SysML equivalent of Class Diagrams
Block Definition Diagrams (BDDs) are essentially exactly the same as Class Diagrams. But, rather than Classes (a software-specific term), BDDs represent SysML Blocks. They represent the kinds of things that can exist, but not individual instances of those things. They include attributes (see: SysML Block Property Types) like how Class Diagrams have properties, and they include behaviors that entities can exhibit, like methods in Class Diagrams.
Block Definition diagrams can also include things like constraints, which may impose rules about valid configurations of block instances; and ports which are places/interfaces wherein interactions may occur.